[libcxx-commits] [libcxx] [libc++] Document our ABI guarantees and what ABI flags exist to modify these guarantees (PR #132615)

[Louis Dionne via libcxx-commits]

================
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+.. _ABIGuarantees:
+
+=======================
+libc++'s ABI Guarantees
+=======================
+
+libc++ provides multiple types of ABI guarantees. These include stability of the layout of structs, the linking of TUs
+built against different versions and configurations of the library, and more. This document describes what guarantees
+libc++ provides in these different areas as well as what options exist for vendors to affect these guarantees.
+
+Note that all of the guarantees listed below come with an asterisk that there may be circumstances where we deem it
+worth it to break that guarantee. These breaks are communicated to vendors by CCing #libcxx-vendors on GitHub. If you
+are a vendor, please ask to be added to that group to be notified about changes that potentially affect you.
+
+ABI flags
+=========
+All the ABI flags listed below can be added to the ``__config_site`` header by the vendor to opt in to an ABI breaking
+change. These flags should never be set by the user. When porting libc++ to a new platform it should be considered by
+the vendor which flags should be enabled, assuming ABI stability is relevant to them. Please contact the libc++ team on
+Discord or through other means to be able to make an informed decision on which flags make sense to enable, and to avoid
+enabling flags which may not be stable. Flags can be enabled via the ``LIBCXX_ABI_DEFINES`` CMake option.
+
+
+Stability of the Layout of Structs
+==================================
+
+The layout of any user-observable struct is kept stable across versions of the library and any options users are allowed
+to change. There are a lot of structs that have internal names, but are none the less observable by users; for example
+through public aliases to these types or because they affect the layout of other types.
+
+There are multiple ABI flags which affect the layout of certain structs:
+
+``_LIBCPP_ABI_ALTERNATE_STRING_LAYOUT``
+---------------------------------------
+This changes the internal layout of ``basic_string`` to move the section that is used for the internal buffer to the
+front, making it eight byte aligned instead of being unaligned, improving the performance of some operations
+significantly.
+
+``_LIBCPP_ABI_NO_ITERATOR_BASES``
+---------------------------------
+This removes the ``iterator`` base class from ``back_insert_iterator``, ``front_insert_iterator``, ``insert_iterator``,
+``istream_iterator``, ``ostream_iterator``, ``ostreambuf_itreator``, ``reverse_iterator``, and ``raw_storage_iterator``.
+This doesn't directly affect the layout of these types in most cases, but may result in more padding being used when
+they are used in combination, for example ``reverse_iterator<reverse_iterator<T>>``.
+
+``_LIBCPP_ABI_VARIANT_INDEX_TYPE_OPTIMIZATION``
+-------------------------------------------------
+This changes the index type used inside ``variant`` to the smallest required type to reduce the datasize of variants in
+most cases.
+
+``_LIBCPP_ABI_OPTIMIZED_FUNCTION``
+----------------------------------
+This significantly restructures how ``function`` is written to provide better performance, but is currently not ABI
+stable.
+
+``_LIBCPP_ABI_NO_RANDOM_DEVICE_COMPATIBILITY_LAYOUT``
+-----------------------------------------------------
+This changes the layout of ``random_device`` to only holds state with an implementation that gets entropy from a file
+(see ``_LIBCPP_USING_DEV_RANDOM``). When switching from this implementation to another one on a platform that has
+already shipped ``random_device``, one needs to retain the same object layout to remain ABI compatible. This flag
+removes these workarounds for platforms that don't care about ABI compatibility.
+
+``_LIBCPP_ABI_NO_COMPRESSED_PAIR_PADDING``
+------------------------------------------
+This removes artifical padding from ``_LIBCPP_COMPRESSED_PAIR`` and ``_LIBCPP_COMPRESSED_TRIPLE``. These macros are used
+inside the associative and unordered containers, ``deque``, ``forward_list``, ``future``, ``list``, ``basic_string``,
+``function``, ``shared_ptr``, ``unique_ptr``, and ``vector`` to stay ABI compatible with the legacy
+``__compressed_pair`` type. ``__compressed_pair`` has historically been used to reduce storage requirements in the case
+of empty types, but has been replaced by ``[[no_unique_address]]``. ``[[no_unique_address]]`` is significantly lighter
+in terms of compile time and debug information, and also improves the layout of structs further. However, to keep ABI
+stability, the additional improvements in layout had to be reverted by introducing artificial padding.
----------------
ldionne wrote:

```suggestion
stability, the additional improvements in layout had to be reverted by introducing artificial padding. This setting removes that artificial padding.
```

Consider making it a separate paragraph after the first sentence?

https://github.com/llvm/llvm-project/pull/132615